Synchronous axial field electrical machine

ABSTRACT

A permanent magnet electrical machine comprises a first substantially planar member ( 9, 10 ) and a second substantially planar member ( 3 ) arranged substantially parallel to the first substantially planar member. The first and second members are rotatable relative to each other about a common axis substantially perpendicular to the planes of the first and second members. The first member ( 9, 10 ) is provided with an annular array of permanent magnets ( 7, 8 ) coaxial with the common axis and configured to provide a substantially axial magnetic field passing through the second member. The second member ( 3 ) is provided with a first annular array of flat coils ( 1 ) coaxial with the common axis and arranged substantially side-by-side in a first layer and with a second annular array of flat coils ( 2 ) coaxial with the common axis and arranged substantially side-by-side in a second layer. The coils ( 2 ) of the second array are offset in a circumferential direction relative to the coils ( 1 ) of the first array.

This invention relates to a synchronous axial field electrical machine.For example, the machine may be a permanent magnet machine or anelectromagnet machine and may be either a generator or a motor.

Synchronous axial field electrical machines are well known, for examplein the form of generators, in which magnets are moved relative toelectrical conductors in order to convert motion into electrical power.

Many different configurations are possible for synchronous axial fieldelectrical machines, the magnets generally being moved relative tostationary conductor coils. The magnets are generally attached to arotating shaft, the rotating assembly being known as a rotor assembly.The stationary arrangement of conductor coils is known as a statorassembly.

In a typical arrangement for a synchronous axial field electricalmachine, the magnets, often in the form of permanent magnets, arearranged radially in an annular array on a pair of parallel softmagnetic steel discs or plates with adjacent magnets on each platealternating in their polarity. Two discs are then mounted for rotationabout a common axis with an air gap between the two discs such thatmagnets on one disc are directly aligned with magnets of oppositepolarity on the other disc. The magnetic flux passes through the air gapbetween opposing magnets and completes magnetic circuits by travellingthrough the discs to adjacent magnets and then through adjacent airgaps.

By soft magnetic material there is meant herein a material which ismagnetisable in a magnetic field, but is not permanently magnetisable.

The stationary conductor coils are flat and are arranged in the air gapbetween the two discs in an annular array. Rotation of the discs causesa fluctuating magnetic field to pass through the conductors andgenerates an electrical current. In practice the stator employsconductor coils having a width similar to the spacing between adjacentmagnets. The effect of this is that each side of a coil will experiencea magnetic field in the opposite direction, causing current to flow in aradially outwards direction on one side of the coil and in a radiallyinwards direction on the other side of the coil. Therefore, at any pointin time, the current will be driven either clockwise or anti-clockwisearound the coil. This is common practice in synchronous axial fieldgenerators.

In order to minimise eddy current losses in the rotor discs it isdesirable to maximise the number of coils in the stator for each magnetpair provided in the rotor. If the coils are simply placed side-by-side,this imposes a limit on the number of coils that can be provided sincethe coil width should be similar to the magnet spacing. A solution tothis problem is to overlap the coils around the stator so that theleft-hand side of one coil is on top of, or beneath, the right-hand sideof an adjacent coil. However, such an arrangement has a number ofdisadvantages in that it is complicated to manufacture and the coils canbe difficult to cool due to the need to support the coils of theexternal faces of the stator.

Clearly, such an electrical machine can be either a generator or a motorand the magnets can be provided either on the rotor or on the stator.

Such an electrical machine, in the form of a motor, is known from U.S.Pat. No. 4,551,645 which illustrates a number of coil configurations.

It is therefore an object of the present invention to provide asynchronous axial field electrical machine which overcomes or at leastameliorates the abovementioned disadvantages.

According to the present invention there is provided a synchronous axialfield electrical machine comprising a first substantially planar memberand a second substantially planar member arranged substantially parallelto the first substantially planar member, the first and second membersbeing rotatable relative to each other about a common axis substantiallyperpendicular to the planes of the first and second members, wherein thefirst member is provided with an annular array of magnets coaxial withthe common axis and configured to provide a substantially axial magneticfield passing through the second member and wherein the second member isprovided with a first annular array of flat coils coaxial with thecommon axis and arranged substantially side-by-side in a first layer andwith a second annular array of flat coils coaxial with the common axisand arranged substantially side-by-side in a second layer, the coils ofthe second array being offset in a circumferential direction relative tothe coils of the first array.

The magnets may comprise permanent magnets and/or electromagnets.

The first planar member may comprise a rotor assembly and the secondplanar member may comprise a stator assembly.

The first planar member may be formed of a single component. That is,the first planar member may not be laminated.

The first planar member may be formed of a soft magnetic material, forexample a steel, such as mild steel.

The magnets and the coils may be disposed at substantially the sameradial distance from the common axis.

The first planar member may comprise first and second coaxial plateswhich are spaced apart from each other. The magnets may be provided onthat face of each one of the first and second plates facing the otherthereof. The first and second plates may be secured together around theperipheral regions thereof. The second member may be arranged in an airgap between the first and second plates.

The first and second arrays of flat coils may be provided on opposingsides of a support member. The support member may be made of anon-magnetic, non-electrically-conducting material, such as glass fibrereinforced plastics material. The support member may be annular and maybe connected to a shaft by way of spokes. Alternatively, the supportmember may be connected to a shaft and may be provided with aperturesfor the passage of cooling air.

The coils may be embedded in a resin material, such as an epoxy resin.

One or more further layers of coils may be provided.

The coils of each layer may be offset by an amount correspondingsubstantially to the pitch of adjacent coils divided by the number oflayers. For example, two layers of coils may be provided, the coils ofone layer being offset relative to the coils of the other layer by anamount corresponding substantially to half the pitch of adjacent coils.

The synchronous axial field electrical machine may be in the form of agenerator or a motor.

For a better understanding of the present invention and to show moreclearly how it may be carried into effect reference will now be made, byway of example, to the accompanying drawings in which:

FIG. 1 is a plan view of part of a stator assembly of one embodiment ofa synchronous axial field electrical machine according to the presentinvention;

FIG. 2 is a section taken along the line A-A shown in FIG. 1;

FIG. 3 is a perspective view of the stator assembly part shown in FIGS.1 and 2 attached to a main shaft to form a stator assembly;

FIG. 4 is a perspective view of a rotor assembly of one embodiment of asynchronous axial field electrical machine according to the presentinvention in the form of a permanent magnet electrical machine, with atop plate partly cut away and the stator assembly omitted for clarity;

FIG. 5 is a perspective view of a synchronous axial field electricalmachine according to the present invention incorporating the rotorassembly of FIG. 4 and the stator assembly of FIGS. 1 to 3, a top plateof the rotor assembly being partly cut away for clarity;

FIG. 6 is a diagrammatic illustration of one embodiment of a synchronousaxial field electrical machine configured as a generator in which apolyphase output is rectified to produce a direct current output; and

FIG. 7 is a diagrammatic illustration of another embodiment of asynchronous axial field electrical machine configured as a generator inwhich one output phase is rectified to produce a direct current output.

FIGS. 1 to 5 show a synchronous axial field electrical machine in theform of a permanent magnet electrical machine and configured as agenerator with permanent magnets provided on a rotor assembly. However,it will be appreciated the machine can readily be modified in a numberof ways. For example the machine can readily function as a motor and thepermanent magnets can readily be provided on a stator assembly. It willfurther be appreciated the permanent magnets can readily be replaced byelectromagnets.

FIGS. 1 and 2 show part of a stator assembly of an electrical generatorcomprising an annular body 3 of glass fibre reinforced plastics materialor other suitable non-magnetic and non-electrically conducting material.Bonded to one side of the annular body 3 is a single layer of flatconductor coils 1 which are in the form of air coils of copper wire, thecoils 1 being arranged substantially side-by-side in an annularconfiguration coaxial with the axis of the annular body 3. Bonded to theother side of the annular body 3 is a further single layer of flatconductor coils 2 which are also in the form of air coils of copperwire. The coils 2 are arranged substantially side-by-side in an annularconfiguration coaxial with the axis of the annular body 3 and at aradius substantially the same as the layer of coils 1. The assembly ofthe annular body 3 and the layers of coils 1 and 2 is embedded in aresin material 4, such as an epoxy or other plastics resin material, toform part of a stator assembly with the resin material providinglocation, protection and electrical insulation for the coils.

As can be seen from FIG. 1, the two layers of flat coils 1 and 2 arecircumferentially offset relative to each other by an amountcorresponding to half the circumferential dimension of the coils, thatis by an amount corresponding substantially to the pitch of adjacentcoils divided by the number of layers.

If desired, the stator assembly can be provided with thin walls at theaxial faces thereof, in order to protect the coils and stiffen thestator assembly, without significantly reducing the ability of the coilsto be cooled by air flow.

As can be seen from FIG. 3, the stator assembly is completed by securingthe stator component of FIGS. 1 and 2 to a main shaft 5 by way of aspoked stator hub 6 which is fastened to the stator component of FIGS. 1and 2 by suitable fastening means. The spokes of the stator hub 6 allowair to pass either side of the stator assembly for cooling purposes.

FIG. 4 shows a rotor assembly which comprises two parallel plates 9 and10 which are secured together around the peripheries thereof by aplurality of spacers 12 such that the plates 9 and 10 are not rotatablerelative to each other. The plates 9 and 10 are rotatably mounted abouta common axis on the main shaft 5 by way of a bearing 11. The plates 9and 10 are each made of a single piece of soft magnetic material such asmild steel.

A plurality of permanent magnets 7 are secured to the plate 9 on thatface thereof facing plate 10, the magnets 7 being arranged side-by-sidein an annular array coaxial with the axis of the main shaft 5 and at aradius corresponding substantially to that of the coils 1 and 2. Themagnets 7 are arranged radially such that poles of opposing polarity areadjacent in adjoining magnets.

A similar plurality of permanent magnets 8 are secured to the plate 10on that face thereof facing plate 9, the magnets 8 being arrangedside-by-side in an annular array coaxial with the axis of the main shaft5 and at a radius corresponding substantially to that of the magnets 7and the coils 1 and 2. The magnets 8 are arranged radially such thatpoles of opposing polarity are adjacent in adjoining magnets and suchthat each magnet 8 faces a corresponding magnet 7 with poles of opposingpolarity opposite each other. Thus the magnets 7 and 8 create asubstantially axial magnetic field in the stator assembly.

FIG. 5 shows the stator assembly of FIG. 3 positioned within the rotorassembly of FIG. 4 with the coils 1 and 2 positioned in an air gap ofpredetermined magnitude between the two annular arrays of magnets 7 and8 which are secured to the plates 9 and 10.

The configuration of the coils 1 and 2 as flat coils and the position ofthe coils lying flat on opposing faces of the annular body 3 allows thecoils to be located close to the permanent magnets 7 and 8 thuspermitting the size of the air gap to be kept to a minimum, while at thesame time presenting substantially the entire surface area of each coilfor cooling purposes. The annular body can be as thick as may berequired for dimensional stability without reducing the ability of thecoils to cool by releasing heat at the exposed face thereof.

The use of a single piece of soft magnetic material for each of theplates 9 and 10, that is the plates are not laminated, has the advantagethat the plates 9 and 10 are economical to produce and are sufficientlystrong to support further components, such as a blade of a wind turbine.However, a unitary construction for the plates 9 and 10 gives rise toeddy current losses within the plates. The eddy current losses are inturn reduced by providing the two layers of flat coils which arecircumferentially offset relative to each other by an amountcorresponding to half the circumferential dimension of the coils.

The machine illustrated in FIGS. 1 to 5 is a three phase machine inwhich the angular separation between adjacent coils is one and one thirdtimes the angular separation between adjacent magnets (the angularseparation being defined as the angle between the centres of adjacentcoils or magnets, as the case may be, measured about the axis ofrotation). Thus, in the illustrated embodiment there are twelve coils 1,twelve coils 2 and sixteen magnets 7 and sixteen magnets 8. The coils 1and 2 are connected such that the phases alternate between the layers ofcoils. Thus, a first phase may be a coil 1, a second phase is then acoil 2 partly overlapping the coil 1 of the first phase, a third phaseis then a coil 1 adjacent to the coil 1 of the first phase and partlyoverlapping the coil 2 of the second phase. The pattern then continueswith the first phase being a coil 2 adjacent to the coil 2 of the secondphase and partly overlapping the coil 1 of the third phase, and so on.

If desired, a machine can be created generating an arbitrary number ofphases by choosing the appropriate ratio of angular separation ofadjacent coils to the angular separation of adjacent magnets.

The electrical machine described above and shown in FIGS. 1 to 5 can bemodified in a number of ways. For example, a number of machines may beprovided on a common axis, one behind the other. Further, the machinecan function as a motor as an alternative to functioning as a generator.Also the permanent magnets can be replaced by electromagnets. In afurther modification the magnets may be provided on the stator assemblyand the coils may be provided on the rotor assembly. Moreover, a singlearray of magnets may be provided between two layers of coils, the coilsof one layer being circumferentially offset relative to the coils of theother layer as described hereinabove.

The number of layers of coils need not be restricted to two and three ormore layers of coils may be provided. The number of phases may be chosenby appropriate selection of the number of layers of coils and the ratioof the angular separation between adjacent coils and adjacent magnets.In such a case, the machine may be a polyphase machine, that is, threeor more phases. However, particularly where the machine is in the formof a generator the alternating current outputs may be rectified toprovide a direct current output.

FIG. 6 is a diagrammatic illustration of a star configuration ofgenerator, in this case producing three phases, although the number ofphases is not important. One example of this is when the number ofphases equals the number of coils. Coils 1 and 2 in the same phase maybe connected in groups in series or in parallel and the alternatingcurrent output of each group is rectified with diodes 13 in order toproduce a direct current output 14.

FIG. 7 is a diagrammatic illustration where each coil 1 or 2, or groupof coils of the same phase, connected in series or in parallel, is orare electrically isolated from the other coils. The alternating currentoutput of a generator is rectified using, for example, a bridgerectifier 15 in order to produce a direct current output 14. The directcurrent output from multiple rectifiers may be connected together. Itshould be noted the arrangement of rectifiers shown in FIG. 6 may beused if desired.

1. A synchronous axial field electrical machine comprising a firstsubstantially planar member (9, 10) and a second substantially planarmember (3) arranged substantially parallel to the first substantiallyplanar member, the first and second members being rotatable relative toeach other about a common axis substantially perpendicular to the planesof the first and second members wherein the first member (9, 10) isprovided with an annular array of magnets (7, 8) coaxial with the commonaxis and configured to provide a substantially axial magnetic fieldpassing through the second member and the second member (3) is providedwith a first annular array of flat coils (1) coaxial with the commonaxis and arranged substantially side-by-side in a first layer and with asecond annular array of flat coils (2) coaxial with the common axis andarranged substantially side-by-side in a second layer, the coils (2) ofthe second array being offset in a circumferential direction relative tothe coils (1) of the first array.
 2. An electrical machine as claimed inclaim 1, wherein the magnets (7, 8) comprise permanent magnets.
 3. Anelectrical machine as claimed in claim 1, wherein the magnets (7, 8)comprise electromagnets.
 4. An electrical machine as claimed in claim 1,wherein the first planar member (9 10) comprises a rotor assembly andthe second planar member (3) comprises a stator assembly.
 5. Anelectrical machine as claimed in claim 1, wherein the first planarmember (9, 10) is formed of a single component.
 6. An electrical machineas claimed in claim 1, wherein the first planar member (9, 10) is formedof a soft magnetic material.
 7. An electrical machine as claimed inclaim 6, wherein the soft magnetic material comprises a steel.
 8. Anelectrical machine as claimed in claim 1, wherein the magnets (7, 8) andthe coils (1, 2) are disposed at substantially the same radial distancefrom the common axis.
 9. An electrical machine as claimed in claim 1,wherein the first planar member comprises first and second coaxialplates (9, 10) which are spaced apart from each other.
 10. An electricalmachine as claimed in claim 9, wherein the magnets (7, 8) are providedon that face of each one of the first and second plates (9, 10) facingthe other thereof.
 11. An electrical machine as claimed in claim 9,wherein the first and second plates (9, 10) are secured together aroundthe peripheral regions thereof.
 12. An electrical machine as claimed inclaim 9, wherein the second member (3) is arranged in an air gap betweenthe first and second plates (9, 10).
 13. An electrical machine asclaimed in claim 1, wherein the first and second arrays of flat coils(1, 2) are provided on opposing sides of a support member (3).
 14. Anelectrical machine as claimed in claim 13, wherein the support member(3) is made of a non-magnetic, non-electrically-conducting material. 15.An electrical machine as claimed in claim 14, wherein the support member(3) is made of glass fibre reinforced plastics material.
 16. Anelectrical machine as claimed in claim 13, wherein the support member(3) is annular and is connected to a shaft (5) by way of spokes (6). 17.An electrical machine as claimed in claim 13, wherein the support member(3) is connected to a shaft (5) and is provided with apertures for thepassage of cooling air.
 18. An electrical machine as claimed in claim 1,wherein the coils (1, 2) are embedded in a resin material (4).
 19. Anelectrical machine as claimed in claim 1, wherein at least one furtherlayer of coils (1, 2) is provided.
 20. An electrical machine as claimedin claim 1, wherein the coils (1, 2) of each layer are offset by anamount corresponding substantially to the pitch of adjacent coilsdivided by the number of layers.
 21. An electrical machine as claimed inclaim 20, wherein two layers of coils (1, 2) are provided, the coils (1)of one layer being offset relative to the coils (2) of the other layerby an amount corresponding substantially to half the spacing betweenadjacent coils.
 22. A permanent magnet electrical machine as claimed inclaim 1 in the form of a generator.
 23. A permanent magnet electricalmachine as claimed in claim 1 in the form of a motor.
 24. An electricalmachine as claimed in claim 1, wherein the coils (2) of the second arrayare offset in a circumferential direction relative to the coils (1) ofthe first array to an extent that the coils (2) of the second array arein partly overlapping relationship with the coils (1) of the firstarray.
 25. An electrical machine as claimed in claim 1, wherein thecoils (1, 2) are connected such that the electrical phases alternatebetween the first and second arrays of coils.
 26. An electrical machineas claimed in claim 1, wherein the ratio of angular separation ofadjacent coils (7,8) in each array to the angular separation of adjacentmagnets (7,8) corresponds to the number of electrical phases.
 27. Anelectrical machine as claimed in claim 1, wherein the steel is a mildsteel.
 28. An electrical machine as claimed in claim 18, wherein theresin material (4) is an epoxy resin